feat(xce-test-e2e-primitives): !cmd shell-escape via system(), feeds $? (task 2.1)

.kiro/specs/xce-test-e2e-primitives/tasks.md

@@ -17,7 +17,7 @@
   - _Depends: 1.1_
 
 - [ ] 2. Shell-escape
-- [ ] 2.1 `!<cmd>` runs a shell command and feeds `$?`
+- [x] 2.1 `!<cmd>` runs a shell command and feeds `$?`
   - In `preprocess_and_run`, when the first non-whitespace char is `!`, run the remainder via `system()`; set `g_last_status = WEXITSTATUS(rc)` (non-zero on `system` failure); a non-zero status is a command failure (subject to 1.1's abort/continue + exit code).
   - Observable: `!true` succeeds and sets `$?`=0; `!false` fails the session and sets `$?`≠0; a script can `!kill ...` / `!bash start.sh ...` to drive node lifecycle.
   - _Requirements: 2.1, 2.2, 2.3, 2.4_

src/controller/testsuite/test.c

@@ -433,6 +433,41 @@ int preprocess_and_run(char* line)
         return 0;
     }
 
+    /* Shell-escape: a line whose first non-whitespace char is `!` runs the
+     * remainder via system(). The shell command's exit status becomes
+     * g_last_status (the `$?` auto-variable); a non-zero status is a command
+     * failure subject to the same abort-vs-continue rules as execute_line. The
+     * remainder is passed raw to the shell (no parse_argv / auto-var
+     * substitution — that is task 3.1). (xce-test-e2e-primitives task 2.1) */
+    if (*p == '!') {
+        const char* rest    = p + 1;
+        int         sysrc   = system(rest);
+        int         status;
+
+        if (sysrc == -1) {
+            /* system() itself failed to run the shell — treat as failure. */
+            status = 127;
+        } else if (WIFEXITED(sysrc)) {
+            status = WEXITSTATUS(sysrc);
+        } else {
+            /* Signaled / abnormal termination — non-zero failure status. */
+            status = 1;
+        }
+
+        g_last_status = status;
+
+        if (status != 0) {
+            g_session_failed = 1;
+            if (!g_continue_on_error) {
+                return 1; /* abort: stop the read loop */
+            }
+        } else {
+            add_history(line);
+        }
+
+        return 0;
+    }
+
     rc = execute_line(line);
 
     g_last_status = (rc < 0) ? (rc) : 0;